A REVIEW OF FISH AND CRAYFISH INTRODUCTIONS MADE IN FINLAND

K. Westman and P. Tuunainen
Finnish Game and Fisheries Research Institute
Helsinki, Finland

ABSTRACT

Since the middle of the last century, at least 15 new fish species and one new crayfish species have been introduced to Finland. In addition, a number of fish strains and hybrids have been imported. The imports of new species have been made in an attempt to increase the production of fishing waters and to create entirely new fishing opportunities; and from the fifties onward to help in solving the particularly difficult problems in fisheries management caused by hydro-construction, water-level regulation and water pollution. Fast-growing species were transferred to promote fish cultivation and hardy, resistant species were introduced in an attempt to prevent diseases.

The following species are known or are thought to be of importance in fisheries management in Finland: the Siberian whitefish (Coregonus peled), lake trout (Salvelinus namaycush), brook trout (Salvelinus fontinalis), splake (S. namaycush × S. fontinalis), carp (Cyprinus carpio), tench (Tinca tinca) and the signal crayfish (Pacifastacus leniusculus). Experimental stockings with peled whitefish and lake trout have given such promising results that extensive stockings for the improvement of stocks have begun. A total of 6 105 000 one-summer and older whitefish juveniles and 53 000 two-year and older lake trout juveniles were stocked in 1980. Trial stockings with the other species continue.

For the moment, the species introduced into Finnish waters do not appear to have caused any harmful effects. The greatest problem in stockings has been that the new species have done poorly in our harsh conditions. The long winter and short growing season has severely limited their growth, reproduction and spread. In the best circumstances some species have reproduced in some stocking waters but no signs of strong increase or spreading have been observed.

This paper deals with the risks connected with the introduction of new species and means of reducing these risks.

RESUME

Depuis le milieu du siècle dernier, au moins Il espèces nouvelles de poissons et une espèce nouvelle d'écrevisse ont été introduites en Finlande. En outre, un certain nombre d'hybrides et de races de poissons ont été importés. L'importation de nouvelles espèces visait à accroître la production des eaux et à créer des possibilités de pêche jusque-là inconnues. A partir des années cinquante, on a surtout cherché par ce moyen à résoudre les difficiles problèmes d'aménagement causés par la construction d'ouvrages hydro-électriques, par la régulation du niveau des eaux et par la pollution aquatique. On a également transplanté des espèces à croissance rapide en vue de promouvoir la pisciculture et des espèces résistantes afin de prévenir les maladies.

Les espèces suivantes sont considérées comme importantes dans l'aménagement des pêches finlandaises: poisson blanc sibérien (Coregonus peled), truite de lac (Salvelinus namaycush), omble de fontaine (Salvelinus fontinalis), splake (S. namaycush × S. fontinalis), carpe (Cyprinus carpio), tanche (Tinca tinca) et écrevisse américaine (Pacifastacus leniusculus). Les résultats des essais de repeuplement avec Coregonus peled et Salvelinus namaycush ont été si prometteurs que l'on a décidé d'utiliser ces deux espèces sur une grande échelle. On a ainsi mis à l'eau en 1980 6 105 000 poissons blancs d'un été ou plus et 53 000 truites de lac de deux ans ou plus. Les essais de repeuplement se poursuivent pour les autres espèces.

Pour le moment, les espèces introduites dans les eaux finlandaises ne semblent pas avoir eu d'effets préjudiciables. Le plus grand problème est qu'elles donnent des résultats assez médiocres dans les conditions particulièrement rudes qui règnent en Finlande. L'hiver est long et la saison de croissance courte, ce qui limite beaucoup leur croissance, leur reproduction et leur propagation. Dans le meilleur des cas, certaines de ces espèces ont réussi à se reproduire mais aucun signe d'accroissement important ou de propagation n'a été signalé.

L'auteur indique quels sont les risques entraînés par l'introduction de nouvelles espèces et quels sont les moyens de les limiter.

1. INTRODUCTION

In Finland, attempts began more than 100 years ago, through introductions of exotic species, at increasing the production of fish beyond that occurring naturally or at the creation of entirely new fishing possibilities alongside those provided by the original fauna. The fact that environments in their natural state are becoming increasingly rare, due to construction and to pollution in the aquatic systems, also has resulted in increased interest in those fish species which appear to be capable of surviving better than the original species in the altered waters as in reservoirs, regulated lakes or eutrophicated water systems, etc. The introduction of fast-growing, disease-restistant fish and crayfish species has also been motivated by the promotion of aquaculture and the prevention of disease.

Many attempts have been made at enriching Finland's natural fish stocks with exotic fish species and strains. Carp (Cyprinus carpio) was brought to our waters for the first time in the 1860s, and even rainbow trout (Salmo gairdneri) will soon have over a century of history in Finland. Experimental activity increased appreciably when the Evo Inland Fisheries and Aquaculture Research Station, located at Lammi in southern Finland, began operations in 1982 (Seligo, 1897; Brofeldt, 1920). From the very beginning, one of the most important functions of the Station was considered to be the acquisition of “the more valuable fish species” for the waters of the Evo State Park. Hatching, raising and stocking trials were carried out with rainbow trout, brook trout (Salvelinus fontinalis), Lake Peipusjärvi whitefish (Coregonus sp.), tench (Tinca tinca) and big and little mouth black bass (Micropterus salmoides and M. dolomieu) (Brofeldt, 1920).

These and other stocking trials with new species at the beginning of this century did not produce any results to speak of. Just as poor were the results of large-scale attempts in the thirties at domesticating the king (i.e., chinook) salmon (Oncorhynchus tsawytscha). By the beginning of the sixties, only two new species of fish, the Galician tench (Tinca tinca spp.) and the brown bullhead (Ictalurus nebulosus) had been successfully acclimatized in a few of the waters in southern Finland. Through continuous stocking, it has also been possible to maintain populations of rainbow trout and carp in natural waters.

The main reason for the almost complete failure of most of the stockings of the exotic species attempted was probably poor planning and haphazard introduction. The characteristics and environmental requirements of species have not generally been known beforehand and were not investigated. Some species have evidently been unable to adapt to our harsh climate. Other reasons for the poor results may be that the stocking water was unsuitable for the species, stockings were on too small a scale, or juveniles of too small a size were used as stocking material.

Since the mid-fifties there has been considerably renewed interest in the possibilities offered by foreign fish species for developing Finnish fisheries. In particular, an attempt has been made to find new species suitable for managing waters which have proved problematic for fisheries owing to construction, water level regulation or pollution. Instead of the “trial and error” method, an attempt has been made to thoroughly investigate the characteristics of new species and their chances of survival in our waters before commencing stocking trials. Promising species have been used for planned trial stockings in carefully selected waters. These stockings have been used to test the possibilities of a species suitable for management purposes. Trials with many species are still underway but some species have proved so promising that it has been possible to begin widespread management stockings.

2. NEW FISH AND CRAYFISH SPECIES

2.1 General

Since the middle of the last century, at least 15 new fish species and one new crayfish species have been imported into Finland for introduction. Table 1 presents selected data on the species used and the success of the stockings.

In addition to new species, a large number of fish strains and hybrids have been introduced into Finland, including hybrid carp (cross between Amur River wild carp and Galician mirror carp), Neva salmon, Dahl River sea trout, Gullspangsälven landlocked salmon, Lake Peipusjärvi whitefish and a number of Swedish salmon strains. A number of different stocks of the exotic species have also been imported, for example, stocks of kamloops, American and Danish rainbow salmon. Stockings of splake (Salvelinus namaycush × S. fontinalis) have also been made in Finland.

Pink (humpback) salmon (Oncorhynchus gorbusha) and chum salmon (O. keta) have appeared in some of our northern rivers (Tenojoki and Näätämöjoki) as well as along the coast of the Gulf of Bothnia and the Gulf of Finland, originating from stockings in the U.S.S.R.

The purpose of this review is to describe briefly the history and results of stockings of exotic fish and crayfish species carried out in Finland.

2.2 Siberian whitefish (Coregonus peled)

The Siberian whitefish or peled whitefish originates from Siberian lakes and rivers. Its eggs were imported into Finland in 1965, and broodfish stocks were established in fish farms as well as in some natural lakes (Tuunainen, 1968). One-summer old juveniles have usually been used for stocking and introduction (Westman et al., 1983) and 5.6 million fish of this age were produced in 1979.

Results from the stockings have usually been good in polyhumous and in eutrophicated waters as well as in Lokka Reservoir (maximum area 417 km²) in Finnish Lapland. The reservoir is polyhumous. When the reservoir was young (in the seventies) stockings with newly-hatched fry also gave good results. Later, one-summer old young were used for stocking. The catch of peled whitefish has been as high as 12 percent (about 10 tons) of the total catch in Lokka Reservoir. In addition to C.peled, C. lavaretus, Salmo trutta and splake, were used for introductions in this reservoir. The zooplankton-feeding C.peled proved to be the only one of these species which has been of any real importance to the fishery. The potential importance of this species in Finland seems to be quite high, but its importance to the fishery in practice is still mainly unknown. Naturally reproducing stocks have been found in two cases.

Comparative studies of the growth, production and food supply of the introduced C. peled and the native whitefish C. muksun, stocked in the same small forest lakes in southern Finland have been made recently (Hakkari et al., 1983; Pruuki, Pursiainen and Westman, 1983).

2.3 Rainbow trout (Salmo gairdneri)

The first stockings of rainbow trout were made with eggs of rainbow trout imported from Germany in 1897. Fry and older fish were released into small lakes and ponds of the Evo district, Lammi, as well as into Lake Saimaa, at the beginning of the 20th century. In the Evo lakes the fish grew to a size of 300–400 g and in the Saimaa area, near Savonlinna, a rainbow trout weighing 3.8 kg was caught. No self-reproducing stocks originated from these stockings and the fish disappeared some years after the stocking (Tuunainen, 1970).

In the sixties, attention was again paid to the rainbow trout. Rainbow trout were introduced into watercourses all over Finland and also into the sea. Introductions were also made in southern and central Finland into small forest lakes, which were first treated with rotenone to eliminate other fish. The purpose of these experiments was to study the ecology of rainbow trout in such conditions, and to investigate their importance in sport fishing (Tuunainen, 1970). A large cultivation programme was also started for food fish production with fish originating from Denmark and later some stocks were also imported from the U.S.A.

To investigate the profitability of the stockings, 17 817 one and two-year old rainbow trout were tagged with Carlin tags in 1963–66. The recapture rate varied remarkably, from 0.3 to 61.1 percent, the average being 4.3 percent. About half of these taggings was carried out in southern Finland in lakes, as well as in the sea area. Of the 7 047 rainbow trout of 13.5–29.5 cm, only 117 were recaptured during the same year and not more than 16 tags were returned during the second year after tagging. During the third year after tagging only one tag was returned. The recapture rate varied from 0.3 to 8.6 percent in the different water areas, higher recapture rates indicating high fishing intensity. The total average catch per one thousand introduced fish was not more than 22 kg. Thus, the life of the stocked fish in this case is not very long and, because of the low recapture rate, the profitability of the stocking remains low. No natural reproduction was found nor have any possibly unfavourable effects on other fish been reported. Later on, rainbow trout gained popularity for put-and-take sports fisheries in many areas (Svärdson and Tuunainen, 1968).

In small Finnish forest lakes, empty of other fish, the results with rainbow trout were fairly good. They were introduced together with brown trout in eight lakes to study competition between the two species (Tuunainen, 1970). In the first and second years, the mean weight of rainbow trout was greater than, and in some cases double, that of the brown trout, but after the third growing season the brown trout were usually heavier than the rainbow trout. The total catches of rainbow trout and brown trout were 1.3–10.6 kg/ha/year. The maximum size of rainbow trout in these lakes was 1.0–1.2 kg, which was reached in 2–4 years. The scarcity of food and small size of the food organisms were the most important factors limiting the growth of trout in the small forest lakes treated with rotenone. The similarity of food taken by rainbow trout and brown trout in the eight lakes was 54–63 percent in July–September, and 73–100 percent at other times.

In some other lakes in which additional food was provided as dry pellets, the fish did better and their growth was also better than in the lakes with natural food only. However, eutrophication of the lakes and oxygen deficiency in the winter was found to be the most important factor limiting this cultivation method. Lakes with additional feeding were established for sport fishing purposes.

Cultivation of rainbow trout as food fish in fish farms and in net cages has been very successful. During the last fifteen years, this type of culture has been developed more than any other branch of fish culture in Finland. The production of rainbow trout for the market in 1979 was 794 t from sea farms and 2 481 t from inland farms, and the production is still growing (Westman et al., 1983).

2.4 Chinook salmon (Oncorhynchus tsawytscha)

Great efforts to introduce chinook salmon into Finland were made in the thirties. Over a four-year period, some 0.65 million eggs were imported from the west coast of the U.S.A. Reared juveniles were stocked in inland waters and in rivers flowing to the Baltic. The results of this activity were poor and the species did not become acclimatized in the waters selected for experiments.

2.5 Sockeye salmon (Oncorhynchus nerka)

Because of the lack of success with chinook salmon, no further Oncorhynchus were imported into Finland for stocking prior to 1966, when a batch of eggs of sockeye salmon was imported from the U.S.A. The species was cultivated at a few fish hatcheries and juveniles were released during the sixties and seventies in small lakes in central Finland. The species is not known to have reproduced in waters in which it has been released. Random small introductions have not met with any remarkable results and stocks of the species are no longer maintained at fish hatcheries in Finland. It remains possible that stocking trials will be reattempted within the next few years.

2.6 Brook trout (Salvelinus fontinalis)

The North American brook trout was brought to Finland for the first time in the last century. The importation of the species from Germany in 1895 was aimed at establishing farming and for stocking trials. Fresh stocking trials were initiated in 1965, this time using imports from the U.S.A.

The species has been reared at several fish farms and introductions have been made specifically in small waters. With a few exceptions, stocking in running waters has met with little success and current interest in the farming of this species is slight. It has been suggested that the stock imported into Finland has become too well adapted to fish hatchery conditions and for this reason is unable to succeed in nature. It is also possible that the environmental requirements of the species are too poorly known and the habitats selected have not been suitable. It is not known for certain whether the species has established self-perpetuating populations in natural waters. Although goals respecting brook trout have not been attained, it is obvious that stocking trials will be continued. In connexion with further attempts, the objectives of introduction, selection of suitable waters and catching recommendations must be given special attention.

2.7 Lake trout (Salvelinus namaycush)

Eyed eggs of lake trout were imported from the U.S.A. to Finland for the first time in 1955. The eggs originated from the Lake Superior stock. The second import took place in 1966 with eggs originating from Lake Opeongo, Canada. The juveniles of lake trout were introduced into lakes all over Finland, and also since 1957 into certain sea areas (Mutenia, Simola and Tuunainen, 1983). Broodfish stocks were established in fish farms from both of these introductions. In 1963, the first young originating from the broodstock were introduced into natural waters, mostly at two and three years of age. Fish from two months to five years old have also been introduced (Nilsson and Dahlström, 1968; Westman et al., 1983).

The positive results of introductions have led to quite extensive stocking of Lake Inari in Finnish Lapland with lake trout (Mutenia, Simola and Tuunainen, 1983). Fish of 5 kg and over have been caught from Lake Inari and from two other places at least.

Natural reproduction may have taken place in two lakes in Finnish Lapland but so far has not been of any practical importance. In Lake Inari, the lake trout catch (10 t in 1980) was about 10 percent of the total catch, and these fish have proved even more important to the fishery because they eat the stunted and low-valued whitefish, which are thereby converted to highly appreciated food fish.

2.8 Splake (Salvelinus namaycush × S. fontinalis)

The first splake cross was made in Finland by the Finnish Fisheries Foundation in 1968. Interest in the species was deepened by the suitability of the species for stocking into regulated lakes as shown in North America. The species has been farmed at a few fish hatcheries. Introductions have been made over recent years using a few thousand two-year or older juveniles, especially in northern Finnish regulated lakes. The species has not reproduced in our waters and the introductions made so far have not produced any remarkable results. The splake farming and stocking trials have been abandoned for the time being.

2.9 Carp (Cyprinus carpio)

The earliest attempts to introduce carp into Finland were made in 1861, but positive results were not obtained until farming and stocking were begun again in the fifties. The present stock of carp originates from the Aneboda Fish Culture Station in Sweden. At first, the aim of the introduction was to see if carp could be cultivated so far north but due to the promising results obtained, the original aim was extended to see if carp would succeed in natural waters and to study its possibilities especially for the management of eutrophicated waters.

Juveniles are raised in earthen ponds, primarily on natural feed. Cultivation has been run in one phase, using the uncontrolled reproduction method (Ahlfors, Kummu and Westman, 1983).

A total of about 138 000 one-year and older juveniles were stocked from 1957 to 1980 in inland and brackish waters between 60° and 68°N. Most of the stocking has been made in southern Finland. In order to study the profitability of carp stockings, two-year and older stocked juveniles were marked with Carlin tags for a total of 13 000 individuals. In inland waters, the rate of return of carp tags has varied between 0 and 32 percent, which, in terms of catch, means 0–336 kg/1 000 individuals stocked (Sormunen et al., 1976; Ahlfors, Kummu and Westman, 1983).

Stockings were not economically profitable although the picture of the survival of stocked juveniles obtained from returned tags is worse than is actually the case, since tags are lost and not all tags were returned. In addition, Finnish fishermen are unaccustomed to fishing for carp.

The best results were obtained with two or three-year old juveniles at least 20 cm in length and weigh 150 g or more. In eutrophicated waters with a great deal of vegetation, carp had an incremental weight gain during the stocking year of up to 1 000–1 500 g. The largest tagged carp caught weighed 5 100 g and the largest untagged 11 900 g.

Under Finnish conditions, carp would evidently not appear to be able to form a reproductive population, since the juveniles do not survive the first winter in natural waters. The best stocking results have been obtained in eutrophicated lakes, where carp, which are able to survive in waters suffering from lack of oxygen, would be of use for the management of eutrophic and eutrophicated waters in south Finland, which have proved difficult to develop for fisheries.

On the basis of trials carried out in Finland over a 30-year period, carp does not appear to cause any damage to the fish stocks, vegetation or general ecosystem of lakes in which it is introduced. Neither does the species appear to detrimentally compete with indigenous fishes by displacing them or by forming hybrids. No dangerous fish diseases or parasites are known to be associated with carp. With carp introductions, the objectives set have been partially realized, the biggest problem having been the lack of adequate catching operations (Ahlfors, Kummu and Westman, 1983).

2.10 “Galician” tench (Tinca tinca spp.)

In the mid-thirties tench of a fast-growing strain related to the tench indigenous to south Finland were imported into Finland from the Aneboda Fish Culture Station in Sweden. The strain originated from Galicia, Poland. The Porla Fish Culture Station raised one to two-summer juveniles for stocking purposes. The culture methods and ponds employed were the same as those used for carp, which were raised simultaneously with the tench (Ahlfors, Kummu and Westman, 1983). Numerous introductions of tench have been made in numerous natural water bodies as far north as latitude 66°30'. The largest specimens recaptured have weighed over one kilogramme. The strain would appear to be of significance especially in nutrient-rich waters in the south suffering from oxygen lack during the winter. No damage by tench either to other fish in the stocked areas or to other forms of aquatic life has been observed. The greatest difficulty in tench stocking seems to be the low value placed on the species which has resulted in a lack of interest in catching and utilizing this fish.

2.11 Smallmouth black bass (Micropterus dolomieu)

A number of attempts have been made to acclimatize this species in Finland, but with poor results. The first importations of smallmouth black bass fry were made in the 1890s from Germany to the Evo district in southern Finland. These stockings made in small forest lakes failed to establish self-reproducing populations. Smallmouth black bass imported from Canada to the Porla Fish Culture Station in 1958 grew well during the first summer but died in the subsequent winter.

A small stock of smallmouth black bass was again imported to Finland from Sweden in 1966. The fish were stocked in a small lake in northern Finland, but did not form a self-reproducing population and the stocked specimens disappeared within a few years.

2.12 Bigmouth black bass (Micropterus salmoides)

This species was introduced from Germany in 1890s to the lake area at Evo, southern Finland, but without any success. No further introductions have been made.

2.13 Signal crayfish (Pacifastacus leniusculus)

Since 1893 numerous but unsuccessful attempts have been made in Finland to control the disastrous crayfish plague (Aphanomyces astaci). No resistant strains of the only native crayfish, Astacus astacus, have developed. In order to restore crayfish production and to improve the earlier important crayfish fisheries, the plague-resistant signal crayfish, Pacifastacus leniusculus, was introduced into Finland for research and trial stockings in 1967 (Westman, 1973). In 1967–74 a total of circa 40 000 juvenile and adult signal crayfish were stocked in 53 crayfish lakes devastated by the plague. The greater part, about 35 500, were newly-hatched juveniles imported from Sweden; the rest were further cultivated, larger juveniles and adults. Adult signal crayfish were imported from the U.S.A. Since 1974, no new introductions have been made, as we wished to have the results of previous stockings before continuing.

Signal crayfish seem to be able to survive and reproduce under Finnish conditions at least up to 63°30'N and to tolerate fishing pressure. Yearly removal of legal-size (10 cm) signal crayfish from some of the experimental lakes does not seem to have caused any harm to the populations or to their renewal. In a few lakes both Pacifastacus and Astacus occur simultaneously, offering excellent possibilities for comparative studies of both species (Westman and Pursiainen, 1979). According to studies made on the same small lake, signal crayfish seem to grow faster and become sexually mature earlier than the native crayfish. Both species seem to prefer similar biotopes and have similar bionomics and life histories. No signs of hybridization have been observed. No harmful effects of the signal crayfish population have so far been observed, either on the Astacus population or on the ecosystem of the lake. However, the niches of both species seem to overlap to such a great degree that competition for living space will probably occur (Westman and Pursiainen, 1983). Signal crayfish show high, but not complete, resistance against the crayfish plague fungus (Unestam, 1973) and if infected specimens are stocked into or migrate to new water areas, they may spread the fungus, thus causing harm to the highly susceptible native Astacus astacus.

3. RISK FACTORS CONNECTED WITH INTRODUCTIONS OF EXOTIC SPECIES

In the last few years the introduction of exotic species has been approached, both in Finland and elsewhere, with increased caution. Several international organizations, including the International Council for the Exploration of the Sea (ICES), the FAO European Inland Fisheries Advisory Commission (EIFAC) and the International Union for the Conservation of Nature and Natural Resources (IUCN), have made recommendations concerning the introductions of new species which advise that tighter controls be adopted for stockings. Registers and reviews of fish and crayfish stockings and their results and effects have also been drawn up under the auspices of, for instance, FAO and EIFAC (Rosenthal, 1976; Welcomme, 1981).

The most intense opposition to introductions has come from those who hold the opinion that stocking with exotic species amounts to the “falsification” of the fauna. This judgement is based on the damage which the transfer of exotic species may cause, such as that which occurred when sparrows were imported to North America or rabbits to Australia. Perhaps the best known examples in Finland have been the spread of the damage caused by mink and muskrat.

What detrimental effects may result from the introduction of exotic fish or crayfish species? The most important may be the following, modified from Rosenthal (1976) and Shafland (1979):

-   uncontrolled, vigorous reproduction and spread of the introduced species, leading to direct or indirect competition with, and eventual elimination of, native species;

-   introduction of new pests, diseases and parasites, harmful to native species and possibly humans;

-   change in behaviour by the exotic species following introduction, thus allowing it to occupy an unexpected ecological niche or even effect unexpected changes in the behaviour of native species;

-   hybridization with closely related native or previously established foreign species;

-   other possible detrimental effects caused by the introduced species to the aquatic flora or fauna, or to such economic interests as fisheries;

-   the growth and development of the introduced species may be reduced because of less favourable environmental conditions than those found in their indigenous area, resulting in worthless, stunted populations which, even if not always directly detrimental, may nonetheless be a great nuisance in the management and utilization of other natural aquatic resources.

Rosenthal (1976), Momot (1978) and Shafland (1979) have reviewed and discussed examples of the deleterious side-effects of introductions (see also Welcomme, 1981).

The exotic fish and crayfish species introduced in Finnish waters do not at the moment appear to have caused any damage. The greatest problem with introductions has been that new species have generally not thrived in our harsh climatic conditions. The long winter and short growing season seem to limit too severely the growth, production and spread of fish and crayfish. Of the species introduced in Finland, so far evidently only the signal crayfish (Pacifastacus leniusculus) has been able to form self-reproducing populations in at least some of the stocking waters; but there has been no observed sign whatever of too vigorous reproduction and spread even of this species.

Under Finnish conditions there seems to be a clear distinction between the detrimental effects of exotic warm-blooded animals living on land and cold-blooded creatures living in water. Mammals are active throughout the year and usually move freely both on land and in the water; while the cold-blooded aquatic fish and crayfish are very passive during the long cold season, and their environment is always limited. In addition, because of their spawning behaviour and habit of swimming in schools, the abundance of fish and crayfish populations can be controlled by fishing and other methods much more easily and effectively than can the abundance of mammal populations.

In warm countries, considerable damage has been caused by the transfer of new aquatic species. These instances should be kept in mind as warnings but, as such, they should not be applied under our much more severe conditions.

4. PROCEDURES TO BE FOLLOWED IN THE INTRODUCTION OF NEW SPECIES

In Finland there is still a great deal of interest in the introduction of exotic species. In the management for fisheries of modified waters or those in which construction has taken place, as well as in fish cultivation, there is adequate reason, to search for and conduct trials with other fish and crayfish species than those which are indigenous to the country. Of central importance here is the question of finding procedures which will reduce the risks attached to the introduction of new species to the minimum.

In planning for the introduction of exotic species, it is of primary importance to remember that it is impossible to predict with any certainty the results of these introductions. Foreign organisms introduced into new ecosystems do not necessarily behave in the same way as they do in their native habitats (of. Shafland, 1979). The short-term effects a new species has on an ecosystem are almost never the same as its long-term impact. The many aspects relating to the introductions of foreign organisms should be carefully evaluated since, if an exotic fish or crayfish species becomes self-perpetuating through natural reproduction, it may be almost impossible to eliminate it.

On the basis of recommendations and suggestions made by ICES, EIFAC, IUCN and a number of experts (Lachner et al., 1970; Hubbs, 1977 and Shafland, 1979) the authors have prepared the following recommendations for procedures to be followed in Finland prior to making a decision regarding new fish and crayfish introductions.

1. The objectives of an introduction should be clearly stated and defined.

2. The introduction must be shown to meet a clear ecological, economical or recreational need; and the exotic species must have the potential to fulfil this need.

3. The new species should fill a vacant or a little used ecological niche.

4. The biology and ecology of the exotic species, its role in its native ecosystem, its relationships with other organisms (especially with fish and crayfish in this ecosystem) and the role of parasites and diseases should be thoroughly investigated by the authorities of the importing country.

5. All proposed introductions should include an evaluation of at least several candidate species to determine which would be the best suited.

6. The results and effects of previous introduction of the exotic or similar species in other areas should be examined.

7. A preliminary assessment should be made of the probable and especially potentially detrimental effects of the exotic on the aquatic environment in general and, in particular, on the native fish and crayfish species and on aquatic vegetation. Evaluations should also be made on the spreading of the exotic, its possible range, its capability of forming self-perpetuating populations, its catchability and its general adaptability to the new environment.

8. All relevant authorities, Institutes, organizations and associations should take part in the preliminary assessment of the introduction, not just the organization proposing the introduction.

9. Preferably only that progeny of the introduced species produced from a broodstock in a closed or otherwise strictly controlled system should be used as material for stocking in natural environments.

10. Restricted trial stockings with cultivated juveniles should be made under controlled conditions in small, isolated waters to prevent escapes.

11. The results of the trial stocking and other, related investigations should be evaluated by a panel of representatives from all agencies involved.

12. Adequate control methods should be available to prevent over-population or undesired spreading of the introduced species.

Fish and crayfish diseases are combatted in Finland by preventing their spread and by control and remedial activities at fish hatcheries and farms. There are no serious fish diseases in Finland. To prevent the spread of disease it is forbidden to import live fish and eggs without the permission of veterinary authorities. Since 1968 the State Veterinary Medical Institute has been adminstering fish disease control, covering almost all major fish farms. In addition, veterinary authorities have far-reaching powers to institute investigations and carry out preventive measures when serious fish diseases are encountered in either cultured or wild fishes.

According to the fifth rule of the International Fisheries Regulations for the Baltic, issued by the International Baltic Sea Fishery Commission (IBSFC), approval of the Commission is needed for the introduction of an exotic species in the Baltic.

Fish and crayfish species considered for introduction into Finland should preferably have the following characteristics:

1. Adapt either to natural waters or to altered waters (artificial, regulated, eutrophicated and polluted lakes, constructed rivers) so well that economically feasible populations would develop.

2. No harmful impact on the aquatic nature and other fields of economic interests, e.g., fisheries.

3. No harmful competititon with native species.

4. No hybridization with native species in natural conditions.

5. Fast-growing.

6. Good food source, either because of large individual size or because it swims in large, easily-caught schools.

7. Primarily vegetarian in food habits.

8. High reproductive potential.

9. Resistant to parasites and diseases occurring in Finland.

10. Non-migratory tendency.

11. Capable of withstanding predation.

12. Resembles native species.

13. Easy to catch even with traditional methods and gear.

14. Good taste when traditionally prepared.

15. Ability to sustain fishing pressure.

16. Disease and parasite free; not a host or intermediate host for diseases and parasites.

17. Does not accumulate heavy metals or pesticides.

18. Suitable for cultivation for production of juveniles for stocking purposes.

Certain special characteristics relating to the enviornment may also be required for each candidate species. For example, imported crayfish species should be genetically resistant to crayfish plague (Aphanomyces astaci).

It is unlikely that any one species will fulfil all of these conditions but the more characteristics it possesses, the more likely it is to survive and flourish in our climate; and the more effective it will be in enriching our fauna in a beneficial way and in finding an ecological niche in its new environment which is in balance with the rest of the ecosystem.

5. REFERENCES

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Brofeldt, P., 1920 Evon kalastuskoeasema. 25-vuotinen toiminta ja tulokset 1892–1917. Suomen Kalatalous, 6:1–141

Hakkari, L. et al., 1984 The food of the native whitefish (Coregonus muksun) and the introduced whitefish (C. peled) stocked in the same small forest lakes in southern Finland. EIFAC Tech.Pap., (42) Vol.1:109–22

Hubbs, C., 1977 Possible rationale and protocol for faunal supplementations. Fisheries, 2(2):12–4

Lachner, E.A. et al., 1970 Exotic fishes and other aquatic organisms introduced into North America. Smithson.Contrib.Zool., 59:1–28

Momot, W., 1978 Invertebrate introductions as possible tool in fish management crayfish. Paper presented at the NABS Workshop, May 10, 1978, Winnipeg, Manitoba. 10 p. (mimeo)

Mutenia, A., O. Simola and P. Tuunainen, 1984 Results of lake trout (Salvelinus namaycush) stockings in Finland in 1957–81. EIFAC Tech.Pap., (42)Vol.2:381–91

Nilsson, N.-A. and H. Dahlstrom, 1968 Harmaanieria (Lake trout). In Kalat, kalavesien hoito ja kalanviljely (Fish, management of fishing waters and fish cultivation), edited by G. Svardson et al. Helsinki, 302 p.

Pruuki, V., M. Pursiainen and K. Westman, 1984 A study of the growth and production of the native whitefish (Coregonus muksun (Pallas)) and the introduced whitefish (C. peled) (Gmelin) stocked in two small forest lakes in southern Finland. EIFAC Tech.Pap., (42) Vol.1:91–108

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Shafland, P.L., 1979 Non-native fish introductions with special reference to Florida. Fisheries, 4(3):18–24

Seligo, A., 1897 Die finnlandische Fischerei-Versuchsstation in Evois. Allg.Fischerei-Ztg., 22:214–5

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Svardson, G. and P. Tuunainen, 1968 Kirjolohi (Rainbow trout). In Kalat, Kalavesien hoito ja kalanviljely (Fish, management of fishing waters, and fish cultivation), edited by G. Svardson et al. Helsinki, 302 p.

Tuunainen, P., 1968 Siika (Whitefish). In Kalat, kalavesien hoito ja kalanviljely (Fish, management of fishing waters, and fish cultivation), edited by G. Svardson et al., Helsinki, 302 p.

Tuunainen, P., 1970 Relations between the benthic fauna and two species of trout in some small Finnish lakes treated with rotenone. Ann.Zool.Fenn., 7:67–120

Unestam, T., 1970 Significance of diseases on freshwater crayfish. In Freshwater crayfish, edited by S. Abrahamsson. Lund, Studentlitteratur, vol. 1:135–50

Welcomme, R.L., (comp.), 1981 Register of international transfers of inland fish species. FAO Fish.Tech.Pap., (213):120 p.

Westman, K., 1973 The population of the crayfish, Astacus astacus L. in Finland and the introduction of the American crayfish Pacifastacus leniusculus Dana. In Freshwater crayfish, edited by S. Abrahamsson. Lund, Studentlitteratur, vol. 1:41–55

Westman, K. and M. Pursiainen, 1979 Development of the European crayfish, Astacus astacus (L.), and the American crayfish, Pacifastacus leniusculus (Dana), populations in a small Finnish lake. In Freshwater crayfish, edited by P.J. Laurent. Thonon-les-Bains, Institut National de la Recherche Agronomique, vol. 4:243–50

Westman, K. and M. Pursiainen, 1984 Introduction of the American crayfish (Pacifastacus leniusculus) in Finland; impact on the native crayfish (Astacus astacus). EIFAC Tech.Pap., (42)Suppl.2:422–6

Westman, K. and P. Tuunainen, 1981 Uusia lajeja vierailta vesilta. Suomen Luonto, 40(Vuosikirja 1981):97–102

Westman, K. et al., 1984 A review of fish stockings in Finland. EIFAC Tech.Pap., (42) Vol.1:252–68

Table 1 Fish and crayfish introductions made in Finland